Ventral anchoring of the conus medullaris: a new surgical technique preventing the recurrence of retethering after surgery for tethered spinal cord

doi:10.21203/rs.3.rs-2430574/v1

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Ventral anchoring of the conus medullaris: a new surgical technique preventing the recurrence of retethering after surgery for tethered spinal cord

https://doi.org/10.21203/rs.3.rs-2430574/v1

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published 28 Apr, 2023

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Objective: Retethering is not an unusual operation for a congenital lumbosacral dysraphic spinal lesion. The present study aimed to assess a new surgical technique for preventing retethering.

Surgical technique: After untethering the spinal cord, the pia mater or scar tissue at the caudal end of the conus medullaris is anchored to the ventral dura mater loosely using 8-0 thread, and the dura mater is closed directly. This technique is called ventral anchoring.

Results: Ventral anchoring was performed in 15 patients (aged 5 to 37 years old, average age: 12.1 years old) between 2014 and 2021. All but one patient showed improvement or stabilization of the preoperative symptoms. No complication directly related to the procedure was observed. Postoperative MRI demonstrated that the dorsal subarachnoid space was restored in 14 patients but was undetectable or absent in three patients on follow-up MRI. No patients have experienced a recurrence of the tethered cord syndrome during the follow-up period.

Conclusion: Ventral anchoring is effective for restoring the dorsal subarachnoid space after untethering the spinal cord. This preliminary study suggested that ventral anchoring has the potential to prevent the postoperative recurrence of tethered cord syndrome in patients with a congenital lumbosacral dysraphic spinal lesion.

Tethered spinal cord

Tethered cord syndrome

Untethering

Lumbosacral congenital anomaly

Retethering of the spinal cord is not uncommon after surgery for a tethered spinal cord (TSC) in patients with previous surgery for a congenital lumbosacral dysraphic spinal lesion. When tethered cord syndrome (TCS) develops, a second operation becomes necessary to correct the tethering. TCS has an incidence of 15% to more than 50% and is now regarded as a time-dependent event [1, 2, 3, 4]. Methods of preventing retethering have been studied extensively. Expansion of the dorsal subarachnoid space via duraplasy is the most common surgical technique [4, 5, 6]. However, even after surgery to correct a TSC recurrence, some patients eventually experience another recurrence requiring further corrective operations [7].

Ventral anchoring of the conus medullaris, a new concept in the treatment of recurrent, postoperative TSC, involves an approach that is diametrically opposed to the traditional one. Unlike standard duraplasty, ventral anchoring pulls the conus medullaris toward the ventral dura mater, thereby securing the dorsal subarachnoid space of the conus medullaris; it positions the conus medullaris away from the dorsal dura mater, keeping it distant from the dorsal dural surface and preventing reattachment of the spinal cord.

The senior author (NM) first applied this simple and effective surgical technique in 2014. The present report describes the surgical technique of ventral anchoring and its outcomes together with lessons learned from its use.

Untethering of a recurrent TSC following repair of a congenital lumbosacral dysraphic lesion was performed 83 times in 72 patients during the last 20 years. Ventral anchoring of the conus medullaris was first performed in 2014 and sporadically applied thereafter. After confirming its usefulness in preserving the dorsal subarachnoid space in 2018, ventral anchoring was performed in 12, consecutive patients who underwent standard untethering surgery. As a result, in total 15 patients (12 female and three male patients, aged 5 years to 37 years; mean age: 12.1 years) underwent ventral anchoring of the conus medullaris by the end of 2021 (Table 1). Postoperative MRI was performed at postoperative week 1. A follow-up MRI was performed at postoperative year 1 and every one to two years as needed.

Surgical technique (Fig. 1)

Surgery for untethering the spinal cord is first carried out in the standard manner. Following the complete circumferential dissection of the conus medullaris, myeloplasty is added at the dissected plane of the conus medullaris whenever possible. Myeloplasty is necessary in most of lipoma cases but only in a few myelomeningocele cases.

Next, the caudal end of the conus medullaris is prepared for ventral anchoring. The pia mater or scar tissue at the terminal part of the conus medullaris is pierced with a needle threaded with 8 − 0 thread. The needle is then pushed through the dura mater on the ventral side, and the thread is tied to form a loop. It is important to anchor the conus medullaris loosely. While the loop should be around 5 to 10 mm long so that the conus medullaris is not tightly fixed to the ventral dura mater, the length of the loop should ultimately depend on the size of the intradural subarachnoid space at the surgical site. If the loop is too short, the conus medullaris will attach to the ventral dura mater. On the other hand, if the loop is too long, the conus medullaris will reattach to the dorsal dura mater, leading to TSC recurrence. In an ideal scenario, postoperative MRI should show the conus medullaris left untouched in the subarachnoid space without contacting the dorsal or ventral dura mater (Fig. 1).

After the ventral anchoring of the conus medullaris, the dura mater is closed tightly. Duraplasty is indicated if securing the dorsal subarachnoid space is difficult via direct dural closure. Intraoperative neurophysiological procedures, such as monitoring the bulbocavernosus reflex (BCR) and pudendal mapping, are performed to provide the surgeon with feedback information relevant to preserving conus medullaris functions [8, 9].

In eight, six, and one patient, the lumbosacral dysraphic lesion was a myelomeningocele, spinal lipoma, and limited dorsal myeloschisis, respectively. Four patients underwent initial surgery to repair the lesion or previously received untethering surgery at another hospital. The duration from the previous surgery ranged from one to 23 years (mean: 9.2 years). Symptoms before untethering were worsening of neurogenic bladder in six patients, deterioration of the moto-sensory function of the lower extremities in five patients, and progression of orthopedic symptoms (scoliosis and the ankle joint deformities) in four patients (Table 1).

Circumferential, complete untethering was achieved in all the patients. The BCR was monitorable in nine patients, was stable in six patients, and was worse but preserved in three patients. None of the patients, including those whose BCR was unmonitorable, experienced postoperative urological deterioration [9]. All but one patient reported improvement or stabilization of the preoperative symptoms during the follow-up period (range: 12–104 months: mean: 48.6 months) (Table 1).

Postoperative MRI demonstrated a ventrally anchored conus medullaris and restored dorsal subarachnoid space in 14 patients (Table 2). In only one patient (case 1), the first to undergo the procedure in the cohort and who showed no improvement after the untethering surgery, postoperative MRI demonstrated that the untethered conus medullaris had again come into contact with the dorsal dura mater. One patient (case 7) showed re-attachment of the conus medullaris to the dorsal dura (re-TSC) on MRI at postoperative year 1 (Fig. 2). In other patients (cases 12 and 14), the conus medullaris shifted dorsally, and the dorsal subarachnoid space became unclear on follow-up MRI at postoperative year 1. The remaining eleven patients had no dorsal re-tethering at the last follow-up MRI performed between postoperative years 1 to 7 (Fig. 3) (Table 2).

No complication directly related to the ventral anchoring was observed, and there was no TCS recurrence during the follow-up period. In addition, there was no case of the conus medullaris touching the ventral dura mater as a result of the ventral anchoring on the last follow-up MRI.

Representative cases

Case #8

The patient, a 37-year-old female, underwent repair for a myelomeningocele soon after birth and received a VP shunt for hydrocephalus. She underwent her first untethering surgery at age 14 years at another hospital after left knee pain developed. At presentation, she had a one-year history of pain in the left sole which slowly worsened and became recalcitrant to medical treatment. She was referred to our hospital for further treatment. Her motor functions were stable although she was paraparetic and required a wheelchair for daily activity. Sensory disturbance was present below the L4 level. MRI revealed a dorsal shift of the spinal cord and the caudal end tethered at L4 (Fig. 4). Marked lumbosacral lordosis associated with TSC was observed [10]. L4 partial laminectomy was performed intraoperatively. Following dural opening, the thick arachnoid membrane was dissected, and the TSC was completely untethered from the dura. The conus medullaris was circumferentially dissected from the dura mater. Then, ventral anchoring was performed between the scar tissue of the conus medullaris and the ventral dura mater. The dorsal dura mater was then closed directly. Her postoperative course was uneventful, and the left sole pain improved markedly. MRI at postoperative week 1 demonstrated the untethered conus medullaris in the middle of the subarachnoid space (Fig. 4).

Case #15

A 16-year-old female patient received surgery for a type 2 spinal lipoma (lipomyelomeningocele) at age 25 days [11]. Subsequently, she underwent two operations for TCS at age 4 years and 7 years at another hospital. Her left ankle joint deformity and weakness gradually deteriorated over the past several years, and sudden weakness developed in the left leg, causing walking difficulty several times a month. She was referred to the senior author (NM) for further treatment. MRI demonstrated a dorsal shift in the position of the spinal cord and a conus medullaris-residual spinal lipoma complex attached to the dorsal dura mater and forming a cystic lesion caudal to the subarachnoid space (Fig. 5). TCS recurrence was diagnosed, and the third untethering surgery was performed. L5 partial laminectomy was performed intraoperatively. Partial resection of the residual lipoma was carried out, and the caudal cyst wall was opened. The conus medullaris was circumferentially untethered from both the rostral and caudal sides. Then, ventral anchoring was performed in the previously described manner. The dura mater was then closed directly. Her postoperative course was uneventful, and the left leg weakness resolved soon after surgery. MRI at postoperative week 1 demonstrated the untethered conus medullaris with a restored dorsal subarachnoid space (Fig. 5).

TCS is a functional disorder caused by stretching of the spinal cord [12]. However, standard untethering surgery with duraplasty often results in TSC recurrence, which requires additional surgery [1, 3, 7]. Sakamoto et al. were the first to attempt retaining the lumbosacral cord ventrally in an operation to correct TSC recurrence [13]. They placed a fine stay suture between the ventro-lateral surface of the conus medullaris and each side of the ventral dura-mater (Fig. 6). They additionally performed duraplasty and laminoplasty to secure a large subarachnoid space dorsal to the conus medullaris. Tubbs and Oakes described placing one or two, transverse stay sutures dorsally to the spinal cord to prevent a dorsal shift while maintaining patency of the dorsal subarachnoid space [14] (Fig. 5). They also additionally performed duraplasty during closure. Compared with these previous procedures, ventral anchoring is simpler and safer. Only a single stay suture is needed in the space created by untethering the spinal cord where there are no neural tissues on the ventral dural surface. The pia-mater or scar tissue at the caudal end of the conus medullaris, then the ventral dura-mater, are pierced by a needle threaded with 8 − 0 thread to decrease vulnerability to neural injury and cerebrospinal fluid leakage.

The concept of keeping the conus medullaris free in the subarachnoid space is important. Instead of expanding the dorsal subarachnoid space via duraplasty, ventral anchoring repositions the conus medullaris by pulling it ventrally to secure the dorsal subarachnoid space. MRI before re-tethering surgery demonstrates an enlarged dural sac and the spinal cord forming a curve and reattaching to the dorsal dura mater [7]. It is thought that the lack of the lamina in spina bifida causes a constant pulsatile load on the dorsal lumbosacral region leading to the dorsal expansion of the dural sac together with dorsal displacement of the spinal cord. The re-shaped curved spinal cord comes into contact with the dorsal dura mater, leading to a recurrence of tethering even after untethering surgery with duraplasty. This is the main reason for the importance of ventral anchoring to prevent spinal cord retethering. In addition, ventral anchoring obviates the need for duraplasty because the dura mater has already been enlarged in most cases.

Ventral anchoring was indicated for all the patients in the present study. It is indicated during the initial corrective surgery if the surgical field is adequate and a large enough subarachnoid space can be secured. We previously applied this technique in an adult case of retained medullary cord with satisfactory outcomes. In certain cases where radical resection of a spinal lipoma is performed, the addition of ventral anchoring is indicated during the initial repair surgery [11, 13, 15].

Ventral anchoring is an orthodox, neurosurgical approach for preventing TSC recurrence and the need for repeated operations. There are other surgical procedures with the same purpose. Spinal cord transection may be indicated in some paraplegic patients [16]. A new surgical procedure called spinal column shortening was recently introduced for intractable, recurrent TCS [17, 18]. While the former is more invasive than ventral anchoring and its indications are limited, the latter will be indicated when the correction of an associated deformed spinal column is necessary.

The present study has some limitations. First, more information from long-term follow-up is indispensable for evaluating outcomes in young children. Second, there is no case-matched control group to compare the surgical outcomes. The utility of ventral anchoring in securing the dorsal subarachnoid space seems clear. However, the present study failed to clarify whether ventral anchoring can prevent the recurrence of TCS over the long term. Third, the length of the thread loop for ventral anchoring needs to be decided intraoperatively. It should be short enough to pull the conus medullaris away from the dorsal dura mater but long enough to avoid ventral tethering of the conus medullaris. In the first patient in the cohort, the loop was too long and caused reattachment of the conus medullaris to the dorsal dura mater. Fourth, the optimal size of the anchoring thread is unknown. In the present study an 8 − 0 nylon thread was used in accordance with Sakamoto’s report [13]. However, in two patients the dorsal subarachnoid space became difficult to detect on MRI during the follow-up period, and in one patient the previously untethered conus medullaris reattached to the dorsal dura mater at postoperative year 1 (Fig. 2). These findings suggested that loosening or severing of the thread loop can occur postoperatively. Tubbs and Oakes used a 5 − 0 thread, but their procedure did not involve piercing the pia mater, unlike Sakamoto’s procedure and our own [14]. The effect of applying ventral anchoring technique with a thicker thread should be examined in the future. Fifth and last, the possibility of ventral, iatrogenic tethering cannot be ruled out completely. The slackness of the loop used in ventral anchoring should minimize this possibility. Future studies, including long-term follow-up, are needed to address these limitations.

Ventral anchoring is a simple and safe surgical procedure when performed by an experienced surgeon and can prevent TSC recurrence. It restores the dorsal subarachnoid space and keeps the conus medullaris away from the dorsal surface of the dura mater. The present preliminary study found that ventral anchoring has the potential to prevent recurrent tethering after corrective surgery for TSC in patients with a congenital lumbosacral dysraphic spinal lesion.

Ethics approval:

This paper was approved by the institutional review board of Tokyo Metropolitan Chidren’s Medical Center (2022-b51).

Consent for publication:

All patient have agreed to present or publish their data for medical purposes.

Availability of data and material:

All figures and tables were prepared for the paper and were not published before.

Conflict of Interest:

There is no conflict of interest in all authors.

Funding:

There is no funding to the paper or any authors.

Competing interest:

There is no competing interest in this study.

Authors’s contribution:

Nobuhito Morota contributed to the study conception and design. Material preparation, and data collection were performed by Makoto Inukai and Masae Kuroha. Material and data analysis were performed by Satoshi Ihara and Nobuhito Morota. The first draft of the manuscript was written by Nobuhito Morota and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

Acknowledgements:

We thank Mr. James Robert Valera for his editorial assistance and professional advice on the manuscript.

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Tables 1 and 2 are available in the Supplementary Files section.

No competing interests reported.

Table1final.xlsx
Table 1: Summary of the clinical course and surgical outcome of the patients MMC: myelomeningocele. LDM: limited dorsal myeloschisis, UT: untethering, NGB: neurogenic bladder
Table2final.xlsx
Table 2: Presence of dorsal subarachnoid space on MRI (+): present, (-): absent, (+/-): unclear

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published 28 Apr, 2023

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Editorial decision: Major revision
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Ventral anchoring of the conus medullaris: a new surgical technique preventing the recurrence of retethering after surgery for tethered spinal cord

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